1. Field of Endeavour
The invention concerns the field of materials technology. It relates to a ceramic thermal barrier coating which is used to coat heavily thermally loaded components, for example rotor blades of a gas turbine.
2. Brief Discussion of Related Art
In order to increase the efficiency of gas turbines, they are run at very high operating temperatures. The components exposed to the hot gases, for example guide vanes and rotor blades or combustion chamber elements, are therefore provided in a known way with thermal barrier coatings (TBC) on their surface in order to achieve higher operating temperatures and/or extend the lifetime of the components. These thermal barrier coatings conventionally consist of a ceramic material, usually of zirconium oxide (ZrO2) stabilized by yttrium oxide (Y2O3), which is applied onto the surface of components often consisting of nickel-based superalloys. In order to improve the bonding of the ceramic coating on the component, adhesive coatings of MCrAlY are often provided between the thermal barrier coating and the surface of the component, where M stands for a metal, specifically for Ni, Fe, Co, or combinations thereof.
It is known to spray the TBC on thermally. Possible methods known for applying these coatings are plasma spraying, for example air plasma spraying (APS), low-pressure plasma spraying (LPPS), vacuum plasma spraying (VPS) or flame spraying, for example high velocity flame spraying (high velocity oxygen fuel HVOF), as well as physical vapor deposition (PVD), for example by means of an electron beam (electron beam physical vapor deposition EB-PVD) (see, for example, U.S. Pat. Nos. 6,352,788 B2 and 6,544,665 B2).
With the aid of EB-PVD, columnar coatings are produced which have an expansion-tolerant grain structure that is capable of expanding or contracting under different loads so that no stresses are generated, which would lead, for example, to flaking of the coatings. The high costs, however, are a disadvantage of this method.
In contrast to this, APS-sprayed TBCs for example have a high degree of inhomogeneities and porosity, which advantageously reduces the heat transfer through the TBC. During operation of a gas turbine, however, the thermal conductivity increases owing to structural modifications, for example grain growth, so that countermeasures need to be implemented in order to achieve sufficient thermal protection. One of these countermeasures, for example, is to spray thicker coatings. Disadvantageously, this is, on the one hand, very expensive and, on the other hand, often not practically feasible. Conventional TBC coating thicknesses are approximately 250-300 μm.
U.S. Pat. No. 6,544,665 B2 therefore proposes to introduce for example Al2O3 (at least 0.1-3 mol. %) into the microstructure of the TBC. The Al2O3 does not bond with the matrix of the ceramic coating; rather, it forms dislocations and therefore prevents the grain growth. This does not, however, have a positive effect on the stress gradients and therefore on reducing the flaking risk of the TBC.